1. Technical Field of the Invention
The present invention relates generally to an apparatus designed to detect a reciprocating motion of an object and/or an amplitude of the reciprocating motion accurately.
2. Background Art
Optical scanners are employed in barcode readers, laser printers, laser radars, etc. which are constructed to sweep a laser beam across a target in one-dimensional direction. As one of this type of optical scanners, an oscillating scanner is known which is designed to achieve the sweeping of a laser beam mechanically by emitting the laser beam to a reflective mirror installed on an oscillating member of the scanner.
The oscillating scanner is required to monitor the orientation of the reflective mirror or the oscillating member in order to determine the time the laser beam is to be emitted to the reflective mirror. As one of methods of determining the orientation of the reflective mirror, a position monitor system has been proposed which uses a permanent magnet and a magnetic sensor made of a pickup coil. The permanent magnet is installed on the oscillating member of the scanner. The magnetic sensor is opposed to a given location defined on a path of swing of the permanent magnet. The magnetic sensor produces an output as a function of an intensity of magnetic field produced therearound for use in determining the time the magnet passes the given location.
In a case where the magnetic sensor is opposed to the center of the path of swing of the magnet, the intensity of the magnetic field generated near the magnetic sensor is maximized when the magnet is at the center of the swing path, while it is minimized when the magnet is at ends of the swing path. At these locations, a rate of a change in magnetic field intensity or the output of the magnetic sensor becomes zero (0). The output of the magnetic sensor differs in polarity between when the magnet is approaching the magnetic sensor and when the magnet is leaving the magnetic sensor. Thus, if the output of the magnetic sensor is defined as being positive when the magnet is approaching the magnetic sensor, when the output of the magnetic sensor is changed from positive to negative, a reference position signal may be produced as indicating the time when the magnetic passes through the center of the swing path.
The output of such a type of magnetic sensor is usually weak in level. It is, thus, necessary to amplify the output of the magnetic sensor. In a case where the output of the magnetic sensor is amplified using an analog amplifier, the amplifier produces an output which varies in level across a reference level cyclically. The output of typical analog amplifiers, however, usually includes an error called an offset which is sensitive to a change in ambient temperature. This may cause the reference level used in the amplifier to be changed by the offset upon a change in environmental condition of use, thus leading to an error in determining the time when the magnetic passes through the center of the swing path.
Further, in a case where the above type of optical scanner is employed in a laser radar for automotive vehicles, the optical scanner is inclined when the vehicle travels on a slope, which may result in a shift of the center of the swing path of the oscillating member. This will cause the amplitude of movement of the oscillating member of the scanner to be changed greatly, thus resulting in an undesirable change in scanned area. Keeping the amplitude of movement of the oscillating member constant requires use of an amplitude sensor, which will result in an unwanted increase in size of the optical scanner.
It is therefore a principal object of the invention to avoid the disadvantages of the prior art.
It is another object of the invention to provide a position determining system capable of determining the position of a reciprocating member such as a reflective mirror of a scanner accurately regardless of a change in environmental condition of use.
It is a still object of the invention to provide a simplified structure of an amplitude monitoring system capable of determining an amplitude of movement of a reciprocating member accurately.
According to one aspect of the invention, there is provided an apparatus designed to determine a position of a reciprocating object such as a scanner mirror of an optical scanner. The apparatus comprises: (a) a sensor opposed to a path along which the object reciprocates, the sensor producing a first signal which varies in level as a function of a distance to the object and is reversed in sign when the object passes the sensor in either of approaching and leaving directions; (b) a signal converting circuit having a first input to which a threshold level that corresponds to a level of the first signal as produced when the object is closest to the sensor is inputted and a second input to which the first signal outputted from the sensor is inputted, the signal converting circuit working to convert the first signal into a second signal that varies in level cyclically across a reference signal level as a function of a difference between a level of the first signal and the threshold level; (c) a comparing circuit comparing a level of the second signal produced by the signal converting circuit with a given reference level to provide a signal indicative thereof; (d) a position signal producing circuit responsive to the signal from the comparing circuit to produce a position signal indicative of the position of the object based on a difference between the second signal and the given reference level; and (e) a reference level updating circuit working to update the given reference level by inputting signals equal in level to each other to the first and second inputs of the signal converting circuit and defining an output of the signal converting circuit as the given reference level. The updating serves to compensate for an error added in the signal converting circuit to the second signal, thereby resulting in an increased accuracy of comparison between the level of the second signal and the reference level.
In the preferred mode of the invention, the reference level updating circuit may be operated in response to a signal inputted from an external.
The reference level updating circuit may be operated cyclically in response to the position signal outputted from the position signal producing circuit.
The object is installed on a movable member of a scanner working to sweep a laser beam.
The object produces a magnetic field. The sensor outputs as the first signal an electric signal which varies in level as a function of a change in magnetic field intensity.
The sensor is opposed to a central portion of the path along which the object reciprocates. The position signal producing circuit produces the position signal indicative of passage of the object through the central portion of the path upon agreement of the level of the second signal with the reference level.
The apparatus may further comprise an initial level determining circuit which works to monitor the first signal for a given period of time to determine an average of a maximum and a minimum level of the first signal as an initial level of the given reference level.
The apparatus may further comprise a level shifting circuit working to shift one of the given reference level and the level of the first signal by a preselected level and an amplitude determining circuit determining a time ratio of a first period of time for which the level of the second signal is smaller than the given reference level to a second period of time for which the level of the second signal is greater than the given reference level to produce a signal indicative of an amplitude of movement of the object for use in controlling the movement of the object.
The reference level updating circuit may update the given reference level by short-circuiting the first and second inputs of the signal converting circuit and defines the output of the signal converting circuit as the given reference level.
The sensor may be opposed to a given location defined on the path along which the object reciprocates. The position signal producing circuit may produce the position signal indicative of passage of the object through the given location when the level of the second signal agrees with the given reference level.
According to the second aspect of the invention, there is provided an apparatus designed to control movement of a reciprocating object which comprises: (a) a sensor opposed to a path along which the object reciprocates, the sensor producing a first signal which varies in level as a function of a distance to the object and is reversed in sign when the object passes the sensor in either of approaching and leaving directions; (b) a signal converting circuit having a first input to which a threshold level that corresponds to a level of the first signal as produced when the object is closest to the sensor is inputted and a second input to which the first signal outputted from the sensor is inputted, the signal converting circuit working to convert the first signal into a second signal that varies in level cyclically across a reference signal level as a function of a difference between a level of the first signal and the threshold level; (c) a comparing circuit comparing a level of the second signal produced by the signal converting circuit with a signal level derived by shifting the reference signal level by a preselected level to provide a signal indicative thereof; and (d) an amplitude determining circuit responsive to the signal from the comparing circuit to determine a time ratio of a first period of time for which the level of the second signal is greater than the signal level to a second period of time for which the level of the second signal is smaller than the signal level to produce a signal indicative of an amplitude of movement of the object for use in controlling the movement of the object.